cmd/vet: validate that time.Format strings are sane

[cellofellow]

vet will let the user know if fmt.Printf strings are incorrect and if the types passed to them are incorrect. It'd be very handy if the also rather specific format for time.Format strings were similarly checked for validity.

[rugginoso]

@cellofellow could you provide some examples of wrong inputs and errors you'll expect?

[agnivade]

fmt.Printf would still print something, albeit not what was intended. But time.Format would simply throw an error. So it's not like a mistake in the time.Format would go unnoticed so easily.

@alandonovan @mvdan